Petroleum Geological Summary
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PETROLEUM GEOLOGICAL SUMMARY RELEASE AREAS W12-6 AND W12-7 SCOTT PLATEAU, BROWSE BASIN AND ROWLEY SUB-BASIN, ROEBUCK BASIN, WESTERN AUSTRALIA
HIGHLIGHTS
Under-explored deep-water frontier situated between the premier hydrocarbon provinces of the Browse and Northern Carnarvon basins
Deep to ultra deep water depths 1,400–5,000 m
Good quality reservoir sandstones, potential source rocks and possible active hydrocarbon systems in nearby wells
A range of potential structural and stratigraphic plays
Release Areas W12-6 and W12-7 are located over the outer, deep-water Scott Plateau in the Browse Basin and the Rowley Sub-basin in the Roebuck Basin. This under-explored part of the North West Shelf lies to the southwest of the Torosa, Brecknock and Calliance gas fields in the Browse Basin, and to the northeast of established oil and gas fields and production infrastructure of the Northern Carnarvon Basin. The Release Areas do not contain any wells, but inboard parts of these areas have good coverage of publicly available 2D seismic reflection data. Nearby wells indicate that there is the potential for active petroleum systems to occur in the area and that a range of structural and stratigraphic plays are possible.
www.petroleum-acreage.gov.au 1 LOCATION
Release Areas W12-6 and W12-7 straddle the southern part of the Scott Plateau, Browse Basin, and the northern part of the Rowley Sub-basin, Roebuck Basin, approximately 350 km off the Western Australian coastline near Broome (Figure 1). Release Area W12-6 is mostly located in water depths of 1,400–2,600 m, deepening to 4,000 m along the northwestern edge of Scott Plateau (Figure 1). The southeastern part of Release Area W12-7 lies in water depths of 1,600– 3,000 m. The far northwestern and northern parts of this Release Area are located over the Argo Abyssal Plain in water depths exceeding 5,000 m (Figure 1).
The graticular block maps and graticular block listings for the Release Areas are shown in Figure 2. Release Area W12-6 comprises 156 full graticular blocks with a total area of approximately 12,874 km2. Release Area W12-7 comprises 154 full graticular blocks with a total area of approximately 12,669 km2.
The Release Areas are situated in an under-explored part of the North West Shelf, between the Torosa, Calliance and Brecknock gas fields in the Caswell Sub-basin of the Browse Basin to the northeast, and the established oil and gas fields and production infrastructure of the Northern Carnarvon Basin to the southwest (Figure 1). There are no exploration wells within the Release Areas.
www.petroleum-acreage.gov.au 2 RELEASE AREA GEOLOGY
Local tectonic setting
The Scott Plateau is a northeast-trending, subsided marginal plateau in the western part of the Browse Basin (Figure 3). Up to 3 km of Mesozoic–Cenozoic rocks have accumulated over Paleozoic and older rocks (Stagg and Exon, 1981). The plateau is separated from the Caswell Sub- basin to the east by the north to north-northeast-trending Buffon–Scott Reef–Brecknock Anticlinal Trend (Figure 3). The eastern part of the Scott Plateau, situated along the boundary with the Caswell Sub-basin, is underlain by the Seringapatam Sub-basin, a major Callovian–Aptian graben overlain by an Aptian–Cenozoic turbidite wedge (Hocking et al, 1994; Hoffman and Hill, 2004). The Scott Plateau is bounded by the Barcoo Sub-basin the southeast, and the Ashmore Platform to the northeast (Hocking et al, 1994).
The Rowley Sub-basin of the Roebuck Basin (Figure 3), which adjoins the south-southwestern boundary of the Scott Plateau, is a major westward-thickening upper Paleozoic–Mesozoic depocentre containing about 9 km of Permo-Carboniferous and older rocks and up to 6 km of Mesozoic–Cenozoic sediments (Smith et al, 1999). The sub-basin is separated from the Beagle Sub-basin and Exmouth Plateau of the Northern Carnarvon Basin to the southwest by the North Turtle Hinge Zone and Thouin Graben, and from the Bedout Sub-basin of the Roebuck Basin to the south by the Bedout High (Figure 3).
Both the Browse and the Roebuck basins evolved through multiple phases of extension, thermal subsidence and uplift during the Paleozoic to Jurassic (Hoffmann and Hill, 2004; Symonds et al, 1994; Struckmeyer et al, 1998; Smith, 1999; Smith et al., 1999) in the lead up to continental breakup and the onset of sea-floor spreading in the Argo Abyssal Plain to the west during the Callovian–Oxfordian (Ludden, 1992).
Structural evolution and depositional history of the area
The Scott Plateau and the Rowley Sub-basin have been poorly studied in comparison to other parts of the North West Shelf and their geological history is largely inferred from studies of the inboard areas of the Browse and Roebuck basins, respectively. Geological summaries of the Scott Plateau are presented by Stagg (1978), Stagg and Exon (1981), and Hoffman and Hill (2004), while Smith (1999) and Smith et al (1999) present an account of the evolution of the Rowley Sub-basin.
Based on work by Stagg and Exon (1981), Symonds et al (1994), Struckmeyer et al (1998), Smith (1999) and Smith et al (1999), the main tectonic phases in the southern Scott Plateau and the northern Rowley Sub-basin (Figure 4) may be summarised as:
Ordovician–Mississippian intracontinental basins
Pennsylvanian–middle/late Permian extension
Middle/late Permian–Late Triassic thermal subsidence
Late Triassic–Early Jurassic compression/transpression
www.petroleum-acreage.gov.au 3 Early–Middle Jurassic extension
Middle–Late Jurassic (Callovian–Oxfordian) continental breakup and uplift
Late Jurassic–Early Cretaceous thermal subsidence
Early Cretaceous–Cenozoic passive margin.
During the Ordovician–Mississippian, cycles of crustal extension and thermal subsidence associated with the separation of Gondwana from Eurasia resulted in the formation of northwest- trending faults and depocentres in the Canning Basin (Kennard et al, 1994) to the southeast of the Release Areas, and possibly in the Browse Basin and Scott Plateau (Stagg and Exon, 1981; Symonds et al, 1994). Successions of alternating marine to terrestrial siliciclastics and shallow marine carbonates, including the Devonian Reef Complex and Fairfield Group in the Canning Basin (Kennard et al, 1994), were deposited. Deposition was interrupted by phases of compressive tectonics, notably during the Early Devonian (Prices Creek Movement) and the Mississippian (Meda Transpressional Movement; Kennard et al, 1994; Smith et al, 1999).
Extension associated with the separation of the Sibumasu Terrane from Gondwana during the Pennsylvanian–Permian resulted in the formation of predominantly northeast-trending structures throughout much of the North West Shelf (Metcalfe, 1988) and the compartmentalisation of the Browse and Roebuck basins into distinct sub-basins (Symonds et al, 1994; Blevin et al, 1998b; Struckmeyer et al, 1998; Smith et al, 1999). Extension occurred predominantly in the lower crust and upper mantle, resulting in half-graben depocentres bounded by major southeast-dipping normal faults (Struckmeyer et al, 1998; Figure 5 and Figure 6). Structures resulting from this extensional event controlled the location and nature of subsequent structural reactivation and deposition in the region (Struckmeyer et al, 1998). Extension gave way to thermal subsidence during the middle/late Permian–Late Triassic. Deposition in the Browse and Roebuck basins was dominated by fluvio-deltaic sediments during the Carboniferous and, in the Permian and Triassic, by marine to fluvio-deltaic siliciclastics (including Permian glacial deposits) and carbonates (Stagg and Exon, 1981; Kennard et al, 2004; Figure 4). The Scott Plateau remained a part of the exposed landmass to the west of the depocentres, and a likely sediment source, until the Middle Jurassic (Stagg, 1978).
The Permo-Triassic thermal subsidence was terminated by regional compressive to transpressive reactivation in the Late Triassic–Early Jurassic (the Fitzroy Movement of the Canning and Roebuck basins), resulting uplift, basin inversion, and formation of large anticlines and unconformities (Stagg and Exon, 1981; Etheridge and O’Brien, 1994; Smith, 1999; Smith et al, 1999). In the Browse Basin, the Buffon–Scott Reef–Brecknock Anticlinal Trend was formed (Kennard et al, 2004).
www.petroleum-acreage.gov.au 4 During the Early to Middle Jurassic, renewed extension in the Browse Basin was generally associated with widespread small-scale faulting and the collapse of the Triassic anticlines (Figure 5). More focused extension over the eastern Scott Plateau resulted in the formation of a deep rift zone, the Seringapatam Sub-basin, which acted as a major sediment sink (Hoffman and Hill, 2004). In contrast, thermal subsidence dominated much of the Roebuck Basin (Smith, 1999; Smith et al, 1999; Figure 6). In both basins, deposition during this time was dominated by fluvio- deltaic to paralic deposition of siliciclastics and coal (Depuch and Plover formations; Figure 4). Moreover, the Browse Basin was situated at the edge of a major volcanic province during this time and extrusive, volcaniclastic and intrusive rocks are commonly encountered within the Jurassic succession (Blevin et al, 1998a; Figure 4).
The Callovian–Oxfordian continental breakup and the onset of sea-floor spreading in the Argo Abyssal Plain resulted in regional uplift and erosion (the Callovian unconformity; Figure 4, Figure 5 and Figure 6), followed by thermal subsidence and widespread marine deposition (the Vulcan Formation; Figure 4, Figure 5 and Figure 6). Significant down-faulting continued in the Seringapatam Sub-basin, resulting in a thick Middle–Upper Jurassic marine succession that includes clastic submarine fans, but sedimentation rates were much lower in the outboard parts of the Scott Plateau (Hoffman and Hill, 2004). In the Roebuck Basin, a condensed section of marine mudstone (Baleine and Egret formations; Figure 4) was deposited until the Early Cretaceous (Smith, 1999; Smith et al, 1999).
After the Valanginian separation of Greater India from Australia, the North West Shelf became a passive margin dominated by marine mudstone and marl deposition (Figure 4). By the Late Cretaceous bathyal conditions were widespread, carbonate deposition replaced siliciclastic deposition (Figure 4), and the shelf edge commenced progradation across the inboard areas of the Browse and Roebuck basins (Stagg and Exon, 1981; Hoffman and Hill, 2004; Kennard et al, 2004). The collision of the Australian and Eurasian plates in the Miocene resulted in accelerated subsidence, rapid deposition of a westward-thinning carbonate wedge, and widespread inversion along existing faults during the Neogene (Shuster et al, 1998; Hoffmann and Hill, 2004; Figure 4, Figure 5 and Figure 6).
www.petroleum-acreage.gov.au 5 EXPLORATION HISTORY
The Caswell Sub-basin, eastern Barcoo Sub-basin, and Leveque and Yampi shelves of the Browse Basin have been well explored and host significant discoveries of gas, condensate and, to a lesser extent, oil (Figure 2). The Scott Plateau and western Barcoo Sub-basin, and the adjoining Rowley Sub-basin of the Roebuck Basin, however, have been sparsely explored. There are no wells within Release Areas W12-6 and W12-7 and only 10 wells within 150 km of the Release Areas (Figure 1), none of which have been discoveries. Initial exploration in this area consisted of isolated exploratory drilling underpinned by a sparse coverage of small 2D seismic grids and regional lines during the 1970s. Seismic data coverage improved considerably during the 1990s and 2000s, when several regional 2D and 3D seismic surveys were completed over the area. This has been accompanied by further drilling activity, most recently at Warrabkook 1 on the Scott Plateau in 2008.
Well control
BARCOO 1 ST2 (1980)
Barcoo 1 was drilled by Woodside Petroleum Development Pty Ltd to test the Upper Triassic and Lower–Middle Jurassic sandstones within a large northeast-trending anticline along the Barcoo Fault Trend (Woodside Petroleum Development Pty Ltd, 1980). Sandstones of Late Jurassic age were a secondary objective. The well is situated in the central Barcoo Sub-basin, Browse Basin, approximately 68 km east of Release Area W12-6, in 720 m of water. The well penetrated a thick Cenozoic, Cretaceous and Jurassic section, and reached a total depth of 5109 mRT in Upper Triassic recrystallised limestone, and is the deepest well drilled in the Browse Basin. The hole was side tracked twice (ST1 from 3,156 m to 3,217 m, ST2 from 3,155 m to 3,278 m) due to a pipe stuck in a tight section of the hole. Original seismic interpretation of the Barcoo structure suggested that both vertical and horizontal closures progressively increased with depth, and progressive thinning of Jurassic strata over the structure was attributed to differential compaction on the flanks of the underlying fault block. Drilling showed that the targeted Callovian unconformity was approximately 600 m shallower than expected and, consequently, closure at this level was small (Woodside Petroleum Development Pty Ltd, 1980). Ditch gas readings were generally low. Of the 96 side-wall cores (SWCs) recovered between 3,284.5 and 4,184 mRT in the Lower Jurassic–Lower Cretaceous succession, fluorescence (mainly cut fluorescence) was recorded in 54. The Lower–Middle Jurassic section (Plover Formation) contained two sandstone zones: the upper zone from 3,830–4,028 mRT with a net thickness of 105 m and 18% porosity, and; the lower zone from 4,105–4,130 mRT with a net thickness of 14 m and 11% porosity. Both units are water wet. Fluid inclusion analysis of the upper sandstone over the depth range 3,830– 3,990 mRT indicates uniformly low Grains with Oil Inclusions (GOITM) values <0.1% (Brincat and Kennard, 2004), and that the level of oil saturation has always been low. The well was plugged and abandoned as a dry hole.
www.petroleum-acreage.gov.au 6 SOUTH GALAPAGOS 1 (2004)
South Galapagos 1 was drilled by Antrim Energy Inc to test the primary objective of Middle–Upper Jurassic to lowermost Cretaceous sandstones of the Vulcan and Plover formations within a four- way dip closure (Antrim Energy Inc, 2005). The well is located in the southern Barcoo Sub-basin of the Browse Basin, approximately 78 km east of Release Area W12-6 in 345 m of water. The nearest wells are Lynher 1, which is 37 km to the east, and Trochus 1, which is 38 km to the east and recorded a weak oil show. The well reached a total depth of 3,636 mRT after encountering water-bearing sandstones within the primary objective with a gross thickness of 462.5 m, net to gross of 44%, and 17% mean porosity in the net sections. Formation tops encountered in the well were mostly 100–200 m higher than predicted, suggesting a velocity anomaly in the upper part of the well, possibly compounded by error in seismic horizon picks (Antrim Energy Inc, 2005). Although sandstones in sidewall cores at 3,117.8–3,455 mRT exhibited very weak direct yellow fluorescence and milky cut, and mature, organic-rich potential source rock units were present, no significant hydrocarbon shows were recorded. Given that the structural integrity of the trap was assessed as high, the absence of hydrocarbons is probably due to a lack of significant charge, reflecting an insufficient volume or maturity of source rock, or a lack of viable migration pathways to the trap (Antrim Energy Inc, 2005). The well was plugged and abandoned as a dry hole.
HUNTSMAN 1 (2006)
Huntsman 1 was drilled by Woodside Energy Ltd to test the primary objective of the Lower Jurassic North Rankin Formation (Depuch and Plover formation equivalents) and secondary objectives within the overlying Lower to Middle Jurassic Murat and Legendre formations (Depuch and Plover formation equivalents; Woodside Energy Ltd, 2007) within the Rowley Sub-basin, Roebuck Basin. The well is located over the Huntsman structure, a tilted fault block compartmentalised by a series of northeast-trending faults, approximately 61 km to the south of Release Area W12-7. The well, drilled in 1,468.6 m of water reached a total depth of 4,375 mRT. The encountered stratigraphy was close to prediction and reservoir sandstones of excellent quality (20–30% porosity, 70–75% net to gross) were encountered in both the primary and the secondary objectives with a total gross thickness of over 1,000 m. However, they were all water saturated and no hydrocarbon shows were recorded. The lack of hydrocarbons is likely to reflect the early timing of main charge relative to that of trap formation or, potentially, a lack of charge due to an absence of mature source rocks or connected hydrocarbons in the area. The well was plugged and abandoned as a dry well.
www.petroleum-acreage.gov.au 7 WARRABKOOK 1 (2008)
Warrabkook 1 was drilled by BHP Billiton Petroleum Pty Ltd to test the exploration potential of the frontier Scott Plateau in the Browse Basin. The well is located approximately 45 km east of Release Area W12-6 in 1,515 m of water, the deepest for an exploration well in Australian waters at the time of drilling (Department of Mines and Petroleum, Petroleum and Environment Division, 2009). The primary exploration objective was to evaluate hydrocarbon prospectivity of the Cretaceous Jamieson Formation sandstones within a four-way closure. The secondary objective was to determine the presence of Plover Formation beneath the Callovian volcanics within a large Jurassic fault block (BHP Billiton Petroleum Pty Ltd, 2008). The well intersected shales and limestones of Barremian–Hauterivian age above approximately 430 m of volcaniclastics before terminating at a total depth of 3,492 mRT. Although gas readings remained very low during drilling, minor gas kicks were associated with zones of significant mud loss within the basal volcaniclastics (BHP Billiton Petroleum Pty Ltd, 2008). The well was plugged and abandoned as a dry hole. Interpretative data for this well was confidential at the time of writing.
Further details regarding wells and available data follow this link: http://www.ret.gov.au/Documents/par/data/documents/Data%20list/data %20list_scott_plateau_AR12.xls
Data coverage
The Release Areas are well covered by recent 2D seismic surveys. Other data sets such as gravity, magnetic and synthetic aperture radar (SAR) data are also available within, and in the vicinity of, the Release Areas.
In 2000, BHP Billiton Petroleum Pty. Ltd. was awarded permits WA-301-P, WA-302-P, WA-303-P, WA-304-P and WA-305-P over the Scott Plateau, immediately to the east of Release Area W12-6. As part of the work program, the HBR2000A survey was completed over the permits, acquiring 18,134 km of 2D seismic reflection data over 302 lines with spacing of 1–15 km (BHP Billiton Petroleum Pty Ltd, 2003). Excellent-quality data was recorded to 10 sTWT with good frequency content within the target interval of 3–5 sTWT. In addition, 17,740 line km of gravity and 17,230 line km magnetic data were acquired and supplemented with the interpretation of 12 SAR scenes from RadarSat. The HBR2000A survey data densely cover the eastern part of Release Area W12-6. BHP Billiton Petroleum Pty Ltd subsequently completed the following surveys within the permits: HBR2001B, acquiring 25,000 line km of airborne hyperspectral (using the CASI sensor) and aeromagnetic data (Ball AIMS, 2002; BHP Billiton Minerals Technology, 2004); HBR2001C, a multibeam bathymetric survey covering an area of 9,500 km2 with a grid resolution of 35–60 m (BHP Billiton Petroleum Pty Ltd, 2002), and; HBR2002A, collecting sea bed piston cores for head space gas and hydrocarbon biomarker analyses and temperature probes (BHP Billiton Petroleum Pty Ltd, 2002; Logan et al, 2008).
www.petroleum-acreage.gov.au 8 Several multi-client 2D seismic surveys cover Release Areas W12-6 and W12-7, particularly the inboard areas. These include: the Deepwater Canning 2D 1998 (Seismic Australia); Browse 1998 and 2002 MC2D (CGG Veritas); Deepwater Canning (Golden Orb) MC2D 2010 (Petroleum Geo- Services ASA, 2011a); North West Shelf (New Dawn) MC2D 2010 (Petroleum Geo-Services ASA, 2011b), and; the North West Shelf Digital Atlas 2010 (Petroleum Geo-Services ASA, 2011c). The Vampire 2D Non-exclusive Seismic Survey (Searcher Seismic Pty Ltd, 2011b) acquired 7,000 km of long-offset lines over the Browse Basin and provides an updated regional well tie for the basin. Lines from the HBR200A and Browse 1998 seismic surveys are currently being reprocessed by Searcher Seismic as the Outer Browse Super-Tie 2D PSTM project (Searcher Seismic Pty Ltd, 2011a).
To view image of seismic coverage follow this link: http://www.ga.gov.au/energy/projects/acreage-release-and-promotion/2012.html#data-packages
PETROLEUM SYSTEMS AND HYDROCARBON POTENTIAL
Sources Upper Jurassic–Lower Cretaceous Vulcan and Baleine formations Lower–Middle Jurassic Plover and Depuch formations Upper Triassic Sahul Group and Bedout Formation equivalents Lower–Middle Triassic Locker Shale, Keraudren Formation and equivalents Lower Permian Grant Group, Poole Sandstone and Noonkanbah Formation equivalents Reservoirs Lower–Upper Cretaceous upper Vulcan, Echuca Shoals, Jamieson and Puffin formations and equivalents Lower–Middle Jurassic Plover and Depuch formations Middle–Upper Triassic Keraudren Formation and equivalents Carboniferous–Permian fluvio-deltaic to shallow marine sandstones Seals Upper Cretaceous–Cenozoic marls and mudstones Upper Jurassic–Lower Cretaceous Vulcan, Echuca Shoals and Baleine formations Lower–Middle Jurassic Plover and Depuch formations Lower–Middle Triassic Locker Shale, Keraudren Formation and equivalents Carboniferous–Permian mudstones and limestones Play Types Miocene inversion anticlines Triassic–Jurassic fault blocks and rollovers Carboniferous–Permian fault blocks and rollovers Cretaceous submarine fan and ponded turbidite sand bodies Callovian sub-unconformity plays Jurassic fluvio-deltaic and shoreline sand bodies Carboniferous–Triassic fluvio-deltaic to marine sand bodies
www.petroleum-acreage.gov.au 9 Source rocks
The presence of active petroleum systems within Release Areas W12-6 and W12-7 is difficult to confirm given the lack of wells. The petroleum systems elements described herein are largely inferred from knowledge developed in the adjoining Barcoo Sub-basin of the Browse Basin and the Roebuck Basin.
As with other areas of the North West Shelf, Lower–Middle Jurassic source rocks are likely to occur within the Plover Formation, being deposited under syn-rift, paralic to shallow marine conditions (Hoffman and Hill, 2004). In the western Barcoo Sub-basin, these source rocks attain considerable thicknesses and TOC content of over 2% is indicated along third- and forth-order sequence boundaries (Blevin et al, 1997, 1998b). The overlying Jurassic lower Vulcan Formation may also contain post-rift, restricted-marine source rocks (Hoffman and Hill, 2004), although the comparatively narrow extent of rifting in this part of the North West Shelf may have limited their distribution (Longley et al, 2002; Keall and Smith, 2004). Regional subsidence and thermal history modelling by Kennard et al (2004) suggests that, if source rocks are present, oil and gas were expelled from the Jurassic rift depocentres on the Scott Plateau during the late Cenozoic. The results of BHP Billiton’s HBR 2002A Seabed Coring Survey (BHP Billiton Petroleum Pty Ltd, 2002) imply Plover and Vulcan Formation source rocks are mature in this region, and elevated GOITM results from the Torosa gas field (North Scott Reef 1, Scott Reef 2A )and Brecknock 1 potentially suggest an early oil charge from the Seringapatam Sub-basin (Kennard et al, 2004). In the Rowley Sub-basin of the Roebuck basin, the Jurassic paralic to marine Depuch and Baleine formations may also contain mature source rocks, particularly within the marine condensed section of the Baleine Formation (Smith et al, 1999). Within Release Areas W12-6 and W12-7, however, the lack of substantial Cenozoic sediment loading implies that Jurassic source rocks may be immature. Pockets of deeply buried Jurassic sediments occur within half graben near the outer edge of the plateau and within the Release Areas, but are likely to lack an effective top seal.
Potential source rocks within the Paleozoic to Triassic succession may include: the Upper Triassic highstand shales seen in Barcoo 1 (Blevin et al, 1998b), the Lower to Middle Triassic transgressive shale and coal within the Locker Shale and Keraudren Formation (Esso, 1994; Smith et al, 1999; Woodside Energy Ltd, 2001) that sourced the gas discovery at Phoenix 1 in the Bedout Sub-basin (Roebuck Basin), and may be presently oil-mature in the outer Rowley Sub-basin (Smith et al, 1999; O’Brien et al, 2003), and; lower Permian marine shales equivalent to the Poole Sandstone, Noonkanbah Formation and upper Grant Group in the Canning and Roebuck basins (Kennard et al, 1994). Within Release Areas W12-6 and W12-7, 2D seismic reflection data indicate the Permian– Carboniferous strata are buried to a depth of 4–6 sTWT beneath the seafloor. As such, the best (though untested) source rock potential in the area may lie within the pre-Jurassic section.
www.petroleum-acreage.gov.au 10 Reservoirs
The Triassic and Jurassic successions are most likely to contain reservoir-quality sandstones. These include: sandy shoreline, shelf and fluvio-deltaic facies of the Triassic Keraudren Formation and Locker Shale in the Rowley Sub-basin (Lipski, 1993; Kennard et al, 1994; Smith et al, 1999) and their equivalents in the Browse Basin; the Lower–Middle Jurassic fluvio-deltaic Plover Formation in the Browse Basin that includes sandstones of excellent quality (e.g. in Huntsman 1; Woodside Energy Ltd, 2007), and; the Lower–Middle Jurassic Depuch Formation in the Roebuck Basin that potentially includes shoreline sand bodies (Smith et al, 1999). Within parts of the Lower to Upper Cretaceous marine succession, submarine fan and ponded turbidite sand bodies have likely reservoir potential, including the Campanian–Maastrichtian Puffin Formation of the Browse Basin (Benson et al, 2004). Finally, sandy fluvio-deltaic to shallow marine facies within the Carboniferous–Permian succession may have reservoir potential.
Seals
Upper Jurassic–Lower Cretaceous marine claystones within the Vulcan and Echuca Shoals formations in the Browse Basin (Blevin et al, 1998b) and within the Baleine Formation in the Roebuck Basin (Smith et al, 1999) are likely regional seals within the Release Areas. Marls and mudstones provide potential seals for Campanian–Maastrichtian ponded turbidites and fans in the Puffin Formation (Benson et al, 2004). Potential intraformational seals may be provided by overbank, paralic, pro-delta and shallow marine mudstones within the Lower–Middle Jurassic Plover Formation in the Browse Basin (Blevin et al, 1998b) and the Depuch Formation in the Roebuck Basin (Smith et al, 1999). Marine claystones, limestones, and pro-delta and paralic mudstones within the Triassic Keraudren Formation and Locker Shale of the Roebuck Basin and their equivalents in the Browse Basin, and similar facies within the Carboniferous–Permian succession, may also form effective local seals (Lipski, 1993; Smith et al, 1999).
Play types
Potential play types have previously been described by Hoffman and Hill (2004) in the Scott Plateau and by Blevin et al (1997, 1998b) in the Barcoo Sub-basin of the Browse Basin. The main likely structural play types within Release Areas W12-6 and W12-7 are: Carboniferous–Permian fault blocks and rollovers, sealed by Triassic claystones, mudstones or limestone; Triassic–Jurassic fault blocks and rollovers, sealed by Upper Jurassic to Lower Cretaceous marine claystones, and; inversion anticlines associated with the Miocene–Holocene tectonism. Potential stratigraphic plays include: Callovian sub-unconformity plays, sealed by Upper Jurassic– Lower Cretaceous marine claystones; Cretaceous submarine fan and ponded turbidite sand bodies sealed by marine claystones and marls; Jurassic fluvio-deltaic channel and shoreline sand bodies sealed by overbank, estuarine, pro-delta and shallow marine mudstones, and; Carboniferous, Permian and Triassic shoreline, shelf and fluvio-deltaic sand bodies sealed by marine claystones, fluvio-deltaic and paralic mudstones, and limestones.
www.petroleum-acreage.gov.au 11 Critical risks
The most significant risks within Release Areas W12-6 and W12-7 are the lack of direct stratigraphic control and the deep water location on the outer margin of the North West Shelf, factors that make the Release Areas an exploration frontier. No significant hydrocarbon shows have yet been recorded in wells near the Release Areas. The geological uncertainty is compounded by the transitional nature of the area between the Browse and the Roebuck basins, such that extrapolations from the better studied inboard parts of these basins may not be valid.
Very weak hydrocarbon indications recorded in some wells (e.g. Barcoo 1, South Galapagos 1 and Warrabkook 1) near the Release Areas suggest that potential source rocks are present within the Release Areas. However, the lack of significant shows in the wells may also indicate insufficient volumes or maturity of the source rocks. Organic-rich rocks within the Lower–Middle Jurassic Plover Formation, the inferred source of the gas in the accumulations along the Buffon–Scott Reef– Brecknock Anticlinal Trend to the northeast of the Scott Plateau, may not be well developed in the Release Areas due to the presence of volcanics and volcaniclastics. Also, the limited extent of Late Jurassic rift depocentres in the area implies that restricted marine oil-prone source rocks of this age (the lower Vulcan and Baleine formations and equivalents) may be absent in places. The outboard location of the Release Areas, away from the major depocentres of the Browse and Roebuck basins and the locus of major Cenozoic sediment loading, suggests that potential source rocks, if present, may not have attained maturity.
Reservoir-quality sandstones are present in the vicinity of the Release Areas, e.g. at Huntsman 1. However, some areas may lack a viable reservoir formation due to the dominance of volcanic and volcaniclastic rocks within the Jurassic to Cretaceous succession, e.g. at Warrabkook 1.
Seal integrity may be a significant risk in the Release Areas. The comparatively thin Cenozoic succession, in combination with ongoing fault reactivation during the Cenozoic, may pose a risk to the integrity of both top and lateral seals, especially in the outboard areas.
www.petroleum-acreage.gov.au 12 FIGURES
Figure 1 Location map of Release Areas W12-6 and W12-7 over the Scott Plateau, Browse Basin, and the Rowley Sub-basin, Roebuck Basin. Exploration wells relevant to Release Areas W12-6 and W12-7 are also shown. Figure 2 Graticular block map and graticular block listings for Release Areas W12-6 and W12-7 over the Scott Plateau, Browse Basin, and the Rowley Sub-basin, Roebuck Basin. Figure 3 Structural elements of the Scott Plateau, Browse Basin, and Rowley Sub-basin, Roebuck Basin, showing the 2012 Release Areas and the exploration wells relevant to Release Areas W12-6 and W12-7. The location of seismic lines in Figure 5, Figure 6 and Figure 7 is also shown. Figure 4 Stratigraphy and hydrocarbon discoveries of the Scott Plateau, Browse Basin, based on the Browse Basin Biozonation and Stratigraphy Chart (Nicoll et al, 2009a), and the Rowley Sub-basin, Roebuck Basin, based on the Canning Basin Biozonation and Stratigraphy Chart (Nicoll et al, 2009b), Smith (1999) and Smith et al (1999). Geologic Time Scale after Gradstein et al (2004) and Ogg et al (2008). Regional seismic horizons after AGSO (2001). Figure 5 AGSO seismic line 128/03 across Release Areas W12-6 and W12-7 over the Scott Plateau, Browse Basin. The location of the seismic line is shown in Figure 3. Regional seismic horizons are shown in Figure 4. Figure 6 AGSO seismic line 120/03 across Release Area W12-7 over the outer Rowley Sub-basin, Roebuck Basin. The location of the seismic line is shown in Figure 3. Regional seismic horizons are shown in Figure 4. Figure 7 Potential petroleum play types of the outer Scott Plateau, Browse Basin, and the outer Rowley Sub-basin, Roebuck Basin, based on AGSO seismic line 128/03. The figure is schematic and does not depict proven plays.
www.petroleum-acreage.gov.au 13 REFERENCES
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